The trend towards “clean energy” and a “hydrogen economy” will be enabled by the efficient supply (e.g. from biomass) and distribution of low cost hydrogen. Fossil fuels can also be converted to hydrogen. The resulting hydrogen is then available for a number of applications, with the fueling of the electrochemical reaction in a fuel cell being one prominent use. Today most commercial hydrogen is produced through catalytic steam reforming of natural gas followed by one or more water gas shift reactors. In the future the gasification of coal and other forms of carbon like lignite probably coupled with some form of carbon dioxide sequestration may also become an important source of hydrogen. However, the low cost production of hydrogen from these resources requires significant technological advances in both hydrogen separation/purification and the efficiency of the Water Gas Shift (WGS) reaction. The WGS reaction:CO+H2OH2+CO2,(ΔG=−41 kJ/mole)is critical to most hydrogen production routes. However, as is expected for exothermic reactions, thermodynamic equilibrium dictates the lower the temperature, the further the reaction can proceed toward the H2 production side. Unfortunately, catalytic activity decreases with temperature and catalysts become more easily inhibited or poisoned, e.g. by sulfur in particular.
Recently, noble metal-based WGS shift catalysts with sufficient volumetric activity and thermal stability have been demonstrated. These easily operate in the 300 to 400° C. range. Such catalysts are well suited to work in concert with a hydrogen separation membrane to both purify the hydrogen produced and to help drive the Water Gas Shift reaction by removing the hydrogen product as it is formed. Thin palladium (Pd), or Pd alloy, membranes, supported on a porous substrate, are suitable for high temperature hydrogen separation and membrane reactor applications, and their application has been discussed in the prior art. Use of such separation membranes avoids the difficulties with present methods of hydrogen separation like Pressure Swing Absorption (PSA). PSA is capital intensive, has a large footprint and high operating costs.
There are important situations where improved sulfur tolerance of both the Water Gas Shift catalyst and the Pd alloy membrane would greatly facilitate the production of fuel cell-grade hydrogen. One is in the small scale production of hydrogen from reformate produced from logistic fuels like gasoline, kerosene, diesel or jet fuel. Reformate is produced by reacting these hydrocarbons with oxygen and/or water in catalytic partial oxidation, autothermal reforming or catalytic steam reforming processes. The resulting “reformate” is a mixture of carbon monoxide and hydrogen, also with some carbon dioxide, water, methane and often nitrogen, etc. For this application, the membrane has be sulfur tolerant enough to handle 20 ppm-wt S-containing fuel that produces on a dry basis about 3 ppmV S reformate. The sulfur content or partial pressure may be lower depending on the amount of N2 from the air introduced into the reforming process. Membranes for hydrogen recovery from such reformates typically require a hydrogen permeance of about 10 m3/(m2-hr-atm0.5) at 350° C., though a higher permeance of 50 m3/(m2-hr-atm0.5) at 350° C. in the absence of sulfur is very desirable, as the higher the permeance, the smaller the membrane area needed. In the presence of ≦6 ppmV sulfur at a partial pressure (as H2S or equivalent) of about 7×10−5 atm or less, slightly lower permeance can be accepted because the cost of sulfur removal can be avoided. Usually for hydrogen recovery, the reformate undergoes a Water Gas Shift Step to adjust the H2/COx ratio, and it is desirable for the Pd-based hydrogen membrane to be compatible with the Water Gas Shift system and especially desirable for the membrane to be suitable for use in a membrane water gas shift reactor.
In portable or vehicular applications, the membrane life should be on the order of 5000 hours, but it should be able to withstand several hundreds of start-up and shutdown cycles with concomitant changes in gas pressure, composition and temperature. It must be tolerant of cycling from −40° C. to 500° C. and operating from ˜200° C. to ˜400° C.
Hydrogen separation membranes could play a key role in the large scale production of hydrogen from “cleaned” coal gas. A representative gas composition for an advanced dry-feed bituminous coal gasifier is: 34% H2, 61% CO, 2% CO2, 3% “other” (dry basis), along with about 200 ppmV sulfur-containing gas, typically an equilibrium mixture of H2S and COS (carbonyl sulfide). A slurry-fed coal gasifier will have proportionally more H2 and CO2 and less CO. For water gas shift, this gas would be blended with about 3 H2O for every CO. Because, for the Water Gas Shift reaction, the ratio of H2O to CO is typically about 3H2O per CO in the feed, a Pd alloy for coal gas water gas shift service should be stable to at least ˜420° C. in the presence of feed gas with partial pressures of, for example, in Atm.: ˜6 H2, 10.8 CO, 0.35 CO2, 0.5 inerts and CH4, 32.3 H2O and about 0.004 atm H2S. As the reaction proceeds, CO+H2O react, providing CO2 and H2. Thus, the membrane also has to be stable to gas with higher CO2 partial pressure. H2 from the water gas shift reaction that is passed through the membrane and is termed “permeate”. Assuming perhaps the permeate side of the membrane involves a counter flow steam sweep gas, then as much as 95% or more of the H2 could diffuse through the membrane, lowering the H2 partial pressure at the gas exit end of the membrane at which the reacted reformate, or “retentate”, appears. The optimum amount of CO conversion achieved and the size of the membrane, and hence the degree of H2 recovery, is typically a matter of site-specific economic calculations. Thus for illustrative purposes, the membrane has to remain stable and permeable to H2 in the presence of (in atmospheres) about 1.22 H2, 0.158 CO, 16.13 CO2, 0.78 inerts and CH4, 31.71 H2O, and about 0.0052 H2S (in equilibrium with COS) in the retentate gas. Such a Pd membrane has to be stable for thousands of hours, preferably 8,000 hrs and more preferably >25,000 hrs, under these harsh operating conditions of >300° C., typically about 400° C. to 420° C., with a peak temperature of about 500° C. or more depending on the system in the presence of 0.005 atm partial pressure or more of sulfur as H2S and carbonyl sulfide. The membrane should have a hydrogen permeance of at least 20 m3/(m2-hr-atm0.5). These operating conditions and the temporal stability required would be a severe challenge to all known Pd-based membranes. Furthermore, any useful hydrogen membrane has to endure, without cracking, system upsets that can result in sudden loss of temperature and pressure. While pure Pd membranes cannot withstand such system upsets, Pd alloys such as described below can.
Pd-atomic hydrogen interactions give rise to a β-phase Pd-hydride, unstable above 295° C. and an α-phase hydride, stable at high temperatures and characterized by markedly lower hydrogen content. At these higher temperatures, the miscibility gap between the α- and β-hydride phases narrows and they coalesce at a critical point around 295° C. and ˜20 atm. The α-β-transition, occurring for pure Pd near room temperature, causes serious alteration in the atom spacing of the metal lattice. The consequent dimensional changes can distort the membrane, making it less mechanically resistant, more brittle and prone to rupture. As a result, the resistance to repeated start-up/shut-down cyclic stresses is low.
Binary addition elements, having in general Face-Centered-Cubic (FCC) structures such as Ag, stabilize the α-hydride phase against the β-hydride phase, reducing the problem of embrittlement, and yield a hydrogen permeability that is greater than that of pure Pd. However, Pd—Ag is rapidly and irreversibly poisoned by sulfur and even with the best sulfur clean-up technologies, there is a reasonable likelihood that a process upset or a change in feedstock will expose the membrane to sulfur. Clearly, neither Pd nor Pd—Ag membranes, in and of themselves, are suitable for use with sulfur-containing feed gas.
A somewhat sulfur-tolerant Body-Centered-Cubic (BCC) phase Pd-40 wt. % Cu alloy with a higher hydrogen permeance than pure Pd has been described in a 2002 DOE report by J. D. Way entitled “Palladium/Copper Alloy Composite Membranes for High Temperature hydrogen Separation from Coal-Derived Gas Streams”. This alloy also avoids the alpha-hydride/beta hydride transition problem that plagues pure Pd. However, the optimum Pd—Cu composition is a BCC β-PdCu phase and is perilously close to the β-PdCu phase/α(FCC)+β(BCC) mixed phase stability boundary. This means that, with an increase in temperature, it can rapidly change structure, from the desirable BCC to the less desirable FCC phase, thus losing its hydrogen permeance and structural integrity. This structural change can be caused by a system upset that increases the temperature beyond the stability of the β phase or because of Pd—Cu segregation occurring over time. Sulfur-containing reformate, rich in CO, at elevated pressures such as described earlier is an ideal environment for this segregation.
Thus, there remains a need for a sulfur tolerant, long life, relatively low cost and high permeance Pd alloy, and a hydrogen generation membrane of such alloy, that does not suffer from either the alpha-hydride/beta-hydride transition problem, having a phase boundary close to the membrane operating point, or metal segregation under sulfur containing reformate with time, under conditions like the production of hydrogen from “cleaned” coal gas via the water gas shift reaction over a sulfur tolerant catalyst such as described above.
Accordingly, a primary advantage of the present invention is the provision of an improved Pd-based alloy, and a membrane of such alloy, that is durable under operating conditions of extended temperature ranges and/or sulfur presence. A further advantage is the provision of such alloy and/or membrane thereof having high H2 permeance. Yet a further advantage is the provision of such alloy and/or membrane thereof being relatively cost effective. A still further advantage is the provision of an alloy and/or membrane thereof, having one or more of the foregoing characteristics and including Cu in the alloy. An even further advantage is the provision of a membrane having such characteristics in the context of incorporation/use with reforming and/or WGS reactors.